Abstract

During the late Miocene east-northeast–directed extension in the Gulf of California extensional province, the western rift margin in northeastern Baja California, Mexico, was segmented at the northwest-striking Matomí accommodation zone. The accommodation zone passed through the northern Puertecitos Volcanic Province and separated pre–6 Ma extension to the north from the unextended region to the south. Pre–6 Ma northeast-side-down displacement is documented across the accommodation zone, which may have undergone dextral oblique-slip motion. The rift margin migrated westward during the latest Miocene or Pliocene, bypassing accommodation-zone structures and incorporating the Puertecitos Volcanic Province into the region of Gulf extensional province deformation. East-northeast– to east-directed extensional deformation is at least post–6 Ma in the northern Puertecitos Volcanic Province and deformation currently continues. Pliocene changes in the Gulf of California spreading center system may have triggered incorporation of the Puertecitos Volcanic Province into the Gulf extensional province as recently as 2–3 Ma. Paleomagnetic analyses of 6.3–6.6 Ma pyroclastic flow deposits show no consistent evidence for rotational deformation, although minor (∼10°–15°) clockwise rotation is possible given anomalous declination directions recorded at some sites. Comparisons with paleosecular variation models and the late Miocene paleopole for stable North America imply no statistically significant rotation relative to geomagnetic north, although minor (∼10°) clockwise rotation is permissible given uncertainties. Geologic relationships show that pre–6 Ma accommodation-zone structures identified in this study did not mark the southern boundary of later rotational deformation documented to the north. The boundary of Pliocene to Holocene rotations may be a broad, diffuse zone of extensional shear encompassing the northeastern Puertecitos Volcanic Province, which accommodated small rotations.

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